I don't think that yahoo would know the difference between the user agent and your browser. I think that your getting a login page because a value is being lost...? Could be that the hidden values are generated and they changed the next time you went to login then the mail system didn't accept the values. Just a guess.

About the form. Autocomplete is an Ie5 feature. If you have Ie5 you can go to tools,Internet Options,Content (Tab), And look for the button that says auto complete. If you turn it on for forms everytime that you use a form it saves the value and if you type in one letter in the form it will bring up all of the previous entries that start with that letter. You can turn it on or off but the form values will still cache so it's a good idea to clear out the cash in the internet options,content,auto complete section about once a month.

I'm not to sure about the name, but I think it's used to define what is in the form. I think that if the form has a name you can do something in the url to put a value in the form field. Like so, http://www.seeki.com/somepage.htm?search=acura.

If Seeki's search form was named search it would probably stick the value (acura) in it.

Well, post refers to the method that the data will be sent. Without knowing your script, I can't say if your sending the data the same way, but you probably are. It's also possible that they wouldn't care either way.

However, the name attribute has me a bit puzzled. I've never used it in a form tag. It may be sending data, just like naming a submit button will. You might want to make a copy of their generated page, change the action to a script of yours, and have the script print out the contents of the various places data get's stuck.

Something like the following might be useful to read whatever data is sent to it and spit it out: